Per the California Department of Education, looks like taking the GED test can cost on average $100.
Also, per the same website, looks like Public Libraries or some of the testing centers have the study materials you would need for free. Sounds like you can study for free, but you might have to pay to take the test...though, it would be a great investment in your future...
This seems like a great website for any questions you may have regarding where and when you can take the tests in California and what the requirements are.
Hope this helps.